1. Field of the Invention
This invention relates to a method of evaluating the tightening condition of a screw member tightened in plastic tightening by a nut runner.
2. Description of the Prior Art
For example, a cylinder head is clamped to a cylinder block by tightening ten or so clamp bolts by a nut runner. The tightening condition of the clamp bolts must be strictly controlled. As described in Japanese Patent Publication No. 55(1980)-35653, it is preferred, in order to obtain a substantially constant axial clamping force, that tightening of a screw member be terminated when the force imparted to the screw member slightly exceeds the elastic limit of the screw member into the plastic range. This manner of tightening a screw member is generally referred to as "plastic tightening". In the plastic tightening, when the force imparted to the screw member before terminating tightening enters far into the plastic range, the fatigue strength of the screw member is lowered and accordingly the tightening condition must be more strictly controlled.
In the above Japanese patent publication, the plastic tightening condition is controlled by a "torque gradient method" in which the instantaneous gradient of a torque-revolution curve while a screw member is tightened, and also whether or not the state of change in the instantaneous gradient signal representing the instantaneous gradient comes to indicate a special point such as a yield point, i.e., whether or not change in the torque increasing rate (ratio of increase in torque to increase in the tightening turning angle) is reduced below a predetermined value, is detected, and the time the tightening of the screw member is to be terminated is determined on the basis of the detection. However, the torque increasing rate changes in the elastic range depending on the manufacturing accuracy of the screw member, physical properties of the member to be clamped by the screw member and the like, and accordingly if the change in the elastic range is similar to that indicating the yield point, the change can be mistaken for the change indicating the yield point to terminate tightening of the screw member before it is sufficiently tightened up.